1. Field of the Invention
The disclosure relates generally to multi-user detection systems and methods thereof in a Code Division Multiple Access (CDMA)-based channel, and more particularly, to multi-user detection systems and methods thereof capable of utilizing multi-user detection to reduce multiple access interference (MAI) in a CDMA common channel.
2. Description of the Related Art
In cellular systems, a base station may communicate with multiple mobile stations at the same time. For a base station receiver used in an uplink transmission (mobile station→base station), it must simultaneously detect signals from all users. Therefore, multi-user detection (MUD) has been widely used in uplink transmission processes. MUD can simultaneously perform data detection for all users and prior to data decision, information for all the users are combined so that the correctness of decision results can be increased. The MUD may be applied to solve the poor performance problem caused by signal overlap, and interference between signals received from multiple users in the same frequency band.
CDMA is one of the popular coding schemes that most systems adopt, which defines a code for each feedback message such that multiple users may transmit predetermined CDMA codes in a CDMA-based uplink common channel. By using CDMA technology, all the users may feedback messages to the base station in a same shared uplink channel to reduce the number of radio resources required for gathering user information. In the uplink channel, users may feedback messages dependant upon the multiple feedback conditions existing and the receiver side may then utilize the orthogonality of codes to distinguish between the feedback conditions. When transmitting multiple CDMA codes in a channel, however, other codes may also utilize the same resources, which would result in multiple access interference (MAI) such that the receiver may detect a failure.
In general, the receiver was unable to estimate the number of users for each group of codes. Note that for each group of codes, signal powers carried therewith are in proportion to the number of users that use the codes. Therefore, the code signal power difference between codes may be very small or very large. When the code signal power difference between the codes is oversized and increases, MAI effect may increase and gradually become obvious. In this case, codes with small signal powers will be easily overlapped by codes with larger signal powers such that code signals with small powers will not be detected when feedback messages have been detected, thereby causing a detection error.
When the power differences betweens code signals are not large, although the situation where codes with small signal powers being easily overlapped by codes with large signal powers may be avoided, however, due to each code being transmitted with similar power strengths, each code may suffer interferences from other groups of codes, thereby causing a burst error while detecting codes.
The aforementioned situation where codes with small powers are easily overlapped by codes with large powers may be avoided by using a multi-stage signal processing technique in a conventional receiver or adjusting the powers of the codes. Using a multi-stage signal processing technique in a conventional receiver, however, will result in signal processing delay, such that the conventional receiver can not process user requirements for wireless channels in time. Moreover, additional resources may be consumed to acquire and control message so as to control the powers of feedback codes.
On the other hand, when the differences among powers of code signals are small, a fixed threshold may be applied to detect signals, wherein the code signal with the strongest code power among the code signals that exceed the threshold is determined to be the detected signal while those code signals with code powers among the code signals that do not exceed the threshold are determined to be noise. However, because all code signals have similar powers, each code may receive interference from other codes such that the detection value after the multi-user detection for each code may be similar to each other. If the detection values exceed the threshold configured by the receiver, all codes can be detected. If the detection values are less than the threshold, all codes can not be detected. Therefore, burst error may occur.
It is therefore a desire to provide multi-user detection capable of solving the MAI problem.